1. Field of the Invention
The invention relates to a brake unit for controlling the brake pressures in the wheel brakes of a road vehicle which is equipped with a hydraulic dual circuit brake system actuable via a hydraulic brake booster.
2. Description of the Prior Art
A brake unit of this type is known from German Patent Specification No. DE 2,453,573 C2.
In this known brake unit, two jointly actuable master cylinders, each assigned to one of the two brake circuits of the vehicle, are arranged in a twin design next to one another in a common housing, in order to achieve a shorter overall length, as compared with a tandem master cylinder. The two master cylinders are designed as static master cylinders, that is to say the brake circuits take the form of closed brake circuits. The brake booster comprises a drive cylinder, the drive pressure space of which can be subjected to an auxiliary pressure proportional to the force with which the driver actuates the brake pedal of the brake system. This auxiliary pressure is derived from an auxiliary pressure source by a brake valve designed as a proportional valve.
The boosted actuating force generated by the drive cylinder of the brake booster acts on a rocker which is mounted on a bearing part, displaceable together with the drive piston of the drive cylinder, so as to be pivotable about an axis extending perpendicularly to the plane marked by the central longitudinal axes of the two master cylinders. The rocker is supported axially by its free ends, for example each via a tappet, on the pistons of the two master cylinders. This support is torque-compensated, in that the ratio L.sub.V /L.sub.H of the effective lengths L.sub.V and L.sub.H of the rocker arms corresponds to the ratio F.sub.H /F.sub.V of the surface F.sub.H of the master cylinder piston limiting the outlet pressure space of the master cylinder assigned to one brake circuit II, and of the surface F.sub.V of the master cylinder piston limiting the outlet pressure space of the master cylinder assigned to the other brake circuit I.
The brake valve is incorporated in the piston of the drive cylinder of the brake booster. Furthermore, the brake booster is designed so that, in the event of a failure of its auxiliary pressure source, the pedal force acts, via the valve body of the brake valve, on the piston of the drive cylinder and, via this, on the rocker. Thus, in the event of such a malfunction, the brake system remains operational, albeit without boosting, and a braking deceleration meeting minimum statutory requirements can still be achieved.
In the known brake unit, although not stated explicitly in DE 2,453,573 C2, but nevertheless absolutely essential to ensure the operating capacity of the known brake unit, the outlet pressure spaces of the two master cylinders, each assigned to a brake circuit, are each delimited by a flange of the master cylinder pistons from a usually annular follow-up space in communication with the brake fluid storage tank in each of the possible positions of the master cylinder pistons. In the position of the master cylinder pistons corresponding to the non-actuated state of the brake system, the outlet pressure spaces of the two master cylinders are also either in communication with the brake fluid storage tank each via a compensating bore shut off after a short initial portion of the pressure build-up stroke of the master cylinder pistons or with the respective follow-up space of the particular master cylinder and therefore likewise with the brake fluid storage tank via a central valve of the particular master cylinder piston which assumes its blocking position after a short initial portion of the pressure build-up stroke of the latter.
A disadvantage of this is that, because of the relatively long closing stroke of such central valves or because of the closing travel required for shutting off the compensating bores, relatively long idle strokes of the pistons have to be allowed for before the build-up of brake pressure in the two master cylinders can finally start. The result of this is an unfavorable brake-pressure/brake-pedal travel characteristic, which is such that the brake pressure increases only slightly at the start of the pedal or piston stroke and experiences an increase proportional to the pedal travel only after relatively long pedal travels have taken place. The response behavior of the brake system is therefore relatively sluggish. In addition, because of production tolerances the brake-pressure/pedal-travel characteristics of such brake units with a twin arrangement of two static master cylinders are subject to considerable spreads which make it difficult to adjust the brake system to a particular brake pressure distribution in an appropriate way.